The basic operation and structure of radio frequency (RF) communication systems, such as cellular radio telephone systems communication systems and land mobile communication systems, are well known in the art. Communication systems typically comprise a plurality of communication units, a predetermined number of base stations (or repeaters) located throughout a geographic region and a controller. The communication units may be vehicle mounted or portable units and comprise either a transmitter or a receiver or both to form a transceiver. The communication unit is coupled to the base station by a communication channel over which RF signals are transmitted and/or received. The controller comprises a centralized call processing unit or a network of distributed controllers working together to establish communication paths for the communication units.
The RF signals typically comprise either analog or digital information signals modulated on a carrier signal at a carrier frequency. Communication units adapted to transmit and/or receive either the analog or digital information signals are characterized as being "dual mode". In a dual mode communication unit consideration is given to combining analog and digital implementations of a particular function performed by the dual mode communication unit to produce an efficient design. One such function performed by the receiver of a dual mode communication unit is automatic frequency control (AFC) of a reference frequency. In general, AFC of a reference frequency is well known in the art.
One problem with implementing AFC in a dual mode receiver is that the characteristics of the modulation require different methods of AFC. The signal received in analog mode, typically a narrow-band FM signal, has no average frequency shift relative to the carrier, the modulation having no DC component. Thus, a simple phase detector or frequency counter can be used to compare a measure of the received signal and a measure of the reference signal, to control the reference signal and tune the receiver and transmitter with respect to the received signal. The signal received in digital mode, conversely, can have an absolute frequency shift with respect to the carrier because the modulation (typically pi/4-shift QDPSK) introduces an arbitrary and possibly accumulating phase shift to the carrier. This requires that a means of removing the effect of the modulation on the digital-mode received signal be provided, to extract a measure of the true carrier for comparison with a measure of the reference signal.
Another problem with implementing AFC in a dual mode receiver is that the response time for digital mode AFC needs to be substantially faster than for analog mode. During a channel change in analog mode, if the two received signals have different carrier frequency offsets, the AFC must be readjusted for the new carrier offset. In analog mode the period of AFC error prior to the completion of the adjustment process results in a slightly degraded audio quality, which can be tolerated for some time; hence a simple AFC correction loop, of low cost and power consumption, is optimum. During a channel change in digital mode, if the relative carrier frequency shifts are significant, the channel may be lost producing a period of lost speech. It is desirable to avoid this; thus, a fast AFC response rate is required in digital mode.
Another problem with implementing AFC in a dual mode receiver is in the switching between the analog and digital modes. When switching from one mode to the other the ending state of the AFC control signal for one mode needs to be used as the beginning state of the AFC control signal for the other mode. With two different methods of generating the AFC control signal, some means needs to be provided for initializing the control signal based on the previous value, when a mode change occurs.
The prior art has addressed the above problems by means of two separate AFC control signal sources, each with a separate means for generating an analog voltage AFC control signal, and a switch which selects between the two. The problem of initializing the digital-mode AFC during the transition from analog to digital mode is addressed by means of an A/D converter which captures the analog voltage generated by the analog-mode AFC control elements, and uses the voltage measured to initialize the digital-mode AFC control elements. The transition from digital mode to analog mode is not addressed, a transient error is permitted which eventually is corrected by the normal action of the analog-mode AFC control elements.
One disadvantage of the prior art solution is that complexity is substantial. Two separate means for generating an analog voltage AFC control signal are needed, and an A/D convertor needs to be provided for the sole purpose of intializing the control voltage for the digital mode. In addition the digital-mode to analog-mode transient is not addressed. Accordingly, there is a need for an improved apparatus and method for automatically controlling a reference frequency in a receiver that receives either an analog information signal or a digital information signal.